Example image of an HVDC transmission system. If more than two converters are linked to a connection, this is known as multi-terminal operation.
Image: Amprion GmbH, Dortmund

Direct and alternating currentHVDC multi-terminal operation

Multi-terminal operation is characterised by the use of at least three converters for HVDC interconnections. This mode of operation therefore forms a precursor to DC grids. Construction using either LCC or VSC technology is generally possible. It only makes sense to install a line-commutated (LCC) variant, however, if one power flow direction is specified throughout the system. If changes in the power flow are planned, a voltage-source (VSC) design is easier to implement.

The project combines energy markets and networks based on the BDEW traffic light. Through a new platform, grid operators can access (load and generation) flexibility, prevent local network congestion and optimise network planning.

Superconductors can protect the grid. Up to a certain temperature, they conduct electricity with no resistance. Only above this so-called transition temperature does their resistance increase exponentially.

The power grid is no longer a one-way street. The expansion of photovoltaic systems and wind turbines and other controlled decentralised systems for electricity generation, storage and use make for new tasks that distribution network must carry out: they must not only distribute electricity, but integrate it as coming from decentralised systems.

In the project NETZ:KRAFT, researchers are developing new grid restoration concepts for future power plant structures. The aim is to integrate renewable energies when restoring supply after a power outage.

In the event of a major fault, or even a complete power outage in the interconnected network, grid operators and operators of generation units have to coordinate the restoration of power supply within their own various grids.

Which smart solutions could help adapt the grids to the new circumstances, allowing them to continue working reliably around the clock even in critical situations? This is the question that researchers are exploring in the “grid-control” project.

Germany has one of the world’s most advanced grids. In the future, this grid will have to integrate more decentralised electricity generation systems on its low voltage level. In addition, electronic devices are becoming more efficient.

If nuclear and coal-fired power plants are turned off temporarily or permanently, then decentralised systems such as wind turbines and photovoltaic systems will have to accomplish the task of providing reactive power for the transmission systems in their stead.

Up until a few years ago, voltage problems were often resolved through expensive conventional grid expansion measures. In the meantime, individual innovative concepts for static voltage stability are often employed.

Due to the expansion of renewable energy, transmission systems and distribution grids are more heavily loaded than they were in the past. Geographic analysis and accurate understanding of the energy flows under different boundary conditions are the core objective of the research project GEOWISOL.

So far, smart grids have failed due to the high costs of smart meters. The grid can still be operated as a smart grid using other technologies, however. Applying these alternative technologies is a declared goal of CheapFlex.

In the joint ISOSTROSE project, the project partners are researching and developing a monitoring system for distribution grid transmission lines. This is aimed at precisely localising earth faults on high-voltage overhead lines in order to enable disturbances in the electricity grid to be quickly eliminated. After the theoretical phase, the scientists will carry out example monitoring in a test area.

With the planned REGEES project, scientists are investigating how the electricity grid can also integrate renewable energy in the future while continuing to work safely. REGEES stands for optimum operating and control strategies for a reliable electrical energy supply system in Germany with a fully integrated infeed of renewable energy sources in a timeframe to 2030. To this end they are developing new operational management and control strategies, whereby if possible they want to fully integrate intermittently generated energy.

Unified planning of the generation and transportation of energy is no longer possible. This means that not only the electricity grid but also the electricity markets now play an important role in ensuring economically efficient energy supplies.

The Green Access project is focussing on creating an intelligent distribution grid. The researchers are primarily working on improving the adaptive monitoring and control algorithms, intelligent control systems and a grid-friendly infrastructure. The scientists are aiming to design the grid so that it can independently adapt itself to future load and infeed changes and varying grid topologies.

In the event of major faults it must be ensured that no photovoltaic systems and wind turbines are shut down because they can contribute to supporting the system by feeding in their power. Within the joint DeF-Neg project, researchers are therefore investigating how measures for stabilising the frequency can be shifted to the distribution grids.

The electrical power supply in Germany is being increasingly dominated by renewable energy generation systems. This is causing conventional power plants to be displaced. But how is this impacting on the grids?

Ancillary services keep the electricity grid stable. With the increased construction of renewable, volatile generation plants, the market for ancillary services is changing. This is because, in comparison with conventional power plants, renewables have some considerable differences.

If the supply security and grid stability usual in Germany are also to be ensured in future, the grid status will need to be monitored constantly. The FiN ("Fühler im Netz" (Sensors in the Grid)) project is investigating the possibility of providing this monitoring using Broadband over Power Line technology.

This project is aimed at analysing and assessing different market and grid connection variants for offshore electricity grids in terms of how the variants impact on the German energy supply system and the superordinate European interconnected power system.

The NEMAR project (network management as a new market role) has resulted from the increasing demands made on decentralised power generators and loads to contribute to ancillary services, system stability and supply security. By jointly coordinating the electricity market participants in the distribution grid, the researchers are aiming to create a new market role.

With high wind energy infeeds, the transmission grid is already reaching the limits of its thermal capacity. Additional transmission capacities will relieve the grid. The DC CTL transmission line (Compact Transmission Line for High Direct Current Voltages) offers a technical solution for providing gas-insulated DC connections.

Team switching: If several switches in direct current (DC) circuits share the tasks this can provide several advantages for the entire system, for example, greater safety, fewer losses and cheaper operating resources.

The SEnCom project aims to reveal security- and reliability-relevant challenges in integrating communication infrastructure in the distribution grids. The project is concerned with analysing both the possibility of external interventions in the information and communication (ICT) systems as well as their impacts on the grid operations.

With the increasing expansion of renewable energies, the medium- and low-voltage distribution grids are reaching their intake limits, particularly in rural areas. This is leading to problems with maintaining the voltage stability and with equipment overloads.

Theoretical and applied scientists and industrial partners are working closely together on developing basic criteria for the stability, reliability, risks and market access of future-proof electricity grids, whereby the focus in on the entire European region and a comprehensive supply with renewable energy sources.

In particular, the ability of rural low-voltage grids to absorb decentralised electricity generation is currently largely limited by the need to maintain the permissible voltages according to DIN EN 50160. The thermal load limits for operating equipment can only be solved by the use of storage systems, down-regulation or by expanding the grid. In contrast, further technical options are available for voltage limit violations.

After which operating time should insulators be replaced? Is it possible to replace them together with other components in order to keep the shutdown periods and costs to a minimum and yet nevertheless use the components in an efficient and safe manner? Are there ways to conduct inspections or special tests that increase the guaranteed service life of the insulators?

The MONA 2030 project is aimed at providing a system-wide comparison of grid-optimising measures. This comparison is intended to make a significant contribution to predictive grid planning that correspondingly takes into account the different facets of the energy generation.

The power grid in Germany is facing major challenges. On the one hand, increasingly more decentralised electricity generators are feeding power into the low- and medium-voltage grids. On the other hand, the transport of energy over long distances is unavoidable in order to transport, for example, energy generated offshore to the load centres. Within this context, the joint KonVeTrO project is investigating how the energy supply system must be expanded in order to meet these challenges.

With the switch to an energy provision that in its ultimate version is intended to be fed almost exclusively from renewable sources, the ‘uncertainties’ provided by these sources are posing considerable challenges. ‘Uncertainty’ in this context means that the output from renewable energies is dominated by factors that can be extremely difficult to predict with reasonable accuracy.

Until now, reliable data for the transmission systems in Europe has not been generally available. That complicates research activities, such as those relating to the optimal design of electricity grids and the network planning of energy systems.

In the STERN research project under the direction of Dr Carsten Pape, researchers are working on depicting the residual load curve as precisely as possible in order to analyse the future electricity supply system. The term residual load refers to the power demanded minus the simultaneous capacity generated by intermittent renewable energy sources. This remaining part of the electricity requirement needs to be met by controllable power plants and imports.

The overarching project outcome is a generalised description method and automation architecture for electrical energy supply grids. The special feature about it is that it meets the required flexibility, adaptability and sustainability in terms of an evolutionary development process for smart grids.

In the joint ReWP project, the researchers are investigating the extent to which wind and solar energy can be used to provide balancing power. This is an essential step in ensuring a high proportion of renewable energies in the overall system.

The use of renewable energy for providing balancing power represents a major challenge on route to increasing the use of renewables in the grid. This is important, however, in order to achieve a stable grid operation. The developers at PV-Regel are therefore researching solutions for photovoltaic systems.

An increasingly decentralised energy production requires new technologies for effective grid protection in the distribution grids. The aim of the research project is to determine the requirements for grid protection with a high proportion of decentralised infeeds.

High-sea wind farms offer significant advantages for the electricity production: the wind conditions remote from the coasts are much more uniform and there is much more space than inland. But how can the wind farms be sensibly integrated into the existing electricity grid?

How could an optimal power grid in Europe look like? When considered in isolation, – i.e. in terms of a theoretical reconstruction – this question can be answered relatively easily. GENESYS2 is considering, however, how the existing power grid can be optimally expanded.

The aim of the project is to make better use of the existing transmission capacities and so reduce the need to expand the transmission grid. The engineers want to achieve this using new algorithms and so-called agent-based methods for monitoring the network.

The increasingly decentralised and temporally fluctuating production of electricity from renewable sources makes it necessary to create additional transport capacity for electrical energy. With critical components for the energy transmission, thermal and electrical phenomena depend heavily on each other and determine the current-carrying capacity.

The aim of the research project is to develop a miniaturised sensor platform that utilises the surface plasmon resonance (SPR) principle, whereby it is intended that it should also be capable of recognising changes in the chemical composition of various fluids both on- and inline.

The expansion of the grid is also presenting the equipment monitoring with new challenges. In the iMonet project, researchers want to develop a measuring method that can read both discrete and distributed sensors in high-voltage systems. One of the aims here is to enable greater grid security even under critical conditions.

In rural areas, overloading can cause grid voltages that exceed the specified tolerances. The grid voltage can be optimally regulated using the kind of controller being developed in the FLOW-R project.

Determining the network state in low-voltage grids as close to real time as possible requires new instruments and measurement methods. Paired with data transmission, these can enable the network state at local network stations to be independently evaluated.

With the expansion of renewable energies, increased amounts of electrical energy will need to be transported in the coming years. In particular the large onshore and offshore wind farms in the sparsely populated north, which have an installed capacity of about 44 GW, along with the load centres in the energy-intensive south make it essential to find new ways for transporting power.

The aim of the IREN2 research project is to develop control systems for microgrids. This is necessary in order to integrate the constantly growing share of renewably generated electricity within the requirements made on the network operations management. One focus of the project is on controlling and scheduling microgrids. Using simulations, the researchers are investigating the limits for ancillary services that topological power plants can bring to the network operations management.